Railroad switch machines are key elements of a railroad installation. A railroad switch machine has to operate over extreme temperatures and is subject to intense shock and vibration. Maintenance of a switch machine may take place in difficult and potentially dangerous conditions, and is a significant expense for a railroad operator because of the large number of switch machines used. Therefore it is important to minimize the required maintenance for the switch machines, while optimizing their reliability and maintaining their performance over time.
Switch machines typically utilize a DC permanent magnet motor to drive a gearbox, whose output is coupled to a mechanical apparatus for moving switch blades of the track. Such switch machines are typically powered by a DC voltage source from a railroad wayside equipment station, and comprise high current contactors powering and driving the DC permanent magnet motor.
The switch machines presented above include, in the gear box, a mechanical torque-limiting clutch. The torque-limiting clutch is adjusted to slip when motor torque exceeds a certain predetermined limit, in order to maintain a good and secured operating of the motor and more generally of the switch machine. One of the main reasons a mechanical clutch is required in switch machines is that track obstacles such as rocks or ice could block normal rail switch motion, in which case the motor could stall. Since the motors in the switch machines presented above are driven by a fixed voltage, the lack of counter electromotive force in the motor windings during a motor stall condition will cause motor current and corresponding motor torque to increase to a very high level. This could damage the motor and/or the switch machine mechanism.
However the use of a torque-limiting clutch has drawbacks. Indeed, the torque-limiting clutch does not provide a precise motor torque control and is also relatively large and expensive. The performance of the mechanical clutch will change with environmental conditions (e.g. temperature and humidity), and will also change over time as the mechanical parts wear and corrosion occurs.
Moreover, periodic maintenance is required for the high current contactors and for the torque-limiting clutch to ensure proper operation of the switch machine.
Besides, it is known from the U.S. Pat. No. 6,366,041 B1 a switch machine comprising switch blades, a motor for moving the switch blades, and a regulation unit to control the electric power supplied to the motor so as to command the movement of the switch blades from an initial position to a final position. In such switch machine, the regulation unit includes high current contactors and a control device of the contactors.
However, in such a switch machine, the regulation unit does not provide a precise control of the motor and a periodic maintenance of the contactors is required. Indeed the contactors are subject to significant mechanical wear because of arcing.